Polyolefin coated pipes are used in off-shore applications for the transportation of hot fluids, e.g. crude oil, and are often installed at the sea-bottom, most often at depths of several hundred meters. For these applications steel pipes are preferred but also fibre reinforced pipes, advanced multilayer pipe constructions made of metal and/or polymer based layers, may be used. At these depths the temperature of the surrounding water is close to 0° C. which leads to extensive heat losses from the transported fluid and significantly reduces flow or causes blockage of the production lines. In order to efficiently pump, e. g. crude oil, it is required that the viscosity is sufficiently low, otherwise a higher pump efficiency or the installation of additional heating units along the pipeline will be necessary.
To meet the insulation requirements on off-shore pipes it has previously been suggested to coat the pipes with an insulating layer of so called syntactic polyolefins, i. e. composite polyolefin/filler materials in which the filler comprises hollow microspheres. As examples of the polyolefin used mention may be made of e. g. linear low density polyethylene, blends of propylene polymers and olefin copolymer elastomers or syntactic polypropylene.
A disadvantage of such syntactic polyolefin coatings is the insufficient mechanical properties of pipes provided with such coatings. At the depths in question the temperature difference between the surrounding water, often having a temperature as low as 0° C., and the inside of pipe, often having a temperature of 100–150° C., put high demands on the mechanical properties. The water pressure on the coating is substantial, and without sufficient compression strength the insulating coating will be compressed to a smaller thickness, thereby reducing its insulating capacity. Also, excellent mechanical properties are required for coated pipes in order to avoid cracking of the coating during installation handling and in service.
The term installation handling used herein means any installation technique such as coiling and uncoiling of the ready made pipelines, welding and other jointing techniques and on-shore or off-shore installation, e. g. off-shore installation at the sea-bottom. Installation of coated pipes, in particular for off-shore applications, involves tough conditions for the protective coating layer, including high stress, substantial elongation, surface damages, notches, impact events, etc, both at low and high temperature conditions and at high hydrostatic pressure. The coating layer is not only protecting the pipeline as such, but is also doing so in a state of high stress and/or at elevated temperatures and pressures, making the coating most sensitive to cracking, e. g. the stresses induced during coiling and uncoiling. During the service life of the coated pipeline, the coating has to protect the pipeline from damages and induced stress and crack formations at conditions close to 0° C., high hydrostatic pressures where a small damage or notch in the coating could propagate into a large crack putting the pipeline as such at risk. With a high dynamic fracture toughness of the coating material no cracks will occur during installation handling and in service.
Another problem is the difficulties in producing syntactic polyolefins. In particular, it is difficult to compound glass microspheres and other hollow spherical fillers into a thermoplastic polymer matrix at low enough shear forces to avoid crushing of the spheres during the process. Also, the thermal conductivity of an effective off-shore pipe insulation needs to be low. When about 15% or more of the spheres in the matrix are crushed, it is difficult to maintain the necessary low level of thermal conductivity. Furthermore, the structural properties of the syntactic polyolefin are also adversely affected. This problem cannot be avoided by adding a larger amount of microspheres since an excessive amount will cause additionally crushed microspheres due to higher forces involved during homogenisation, i. e. >15%, initiates cracks and further deteriorates the mechanical properties.
European Patent Application no EP-A-473 215 discloses polyolefin syntactic foams for pipeline insulation use, wherein microspheres that have been treated with a chain scission agent are added to a fluid stream of short chain polypropylene or polybutylene to form a syntactic foam insulative material. This method is taught as useful for producing material of a low thermal conductivity. However, the plastic starting materials taught for use therein are generally not optimal for submarine pipe insulation, because while the short chain polypropylene or polybutylene affords low breakage of the microspheres, the method requires the presence of a chain scission agent, coated on the microspheres. The chain-scission agent is employed to cause a narrowing of the molecular weight distribution of the polyolefin. Without the presence of this agent, the finished insulation would be unacceptable for use as off-shore pipe insulation, because it would not be hydrostatic pressure resistant, abuse resistant, or creep resistant.